Because I want to retain the the functionality & familiarity of the the Hogarthian Long-Hose & Bungee'd necklaced rbackup that I've been taught since beginning wreck penetration classes with my SE Asia dive mates nearly six years ago --and keeping that configuration and deployment on demand is of paramount importance with my team. . .
(btw, How do you switch out regulators on conventional sidemount tanks --on-the-fly-- while piloting a scooter??? Not a problem or issue with the Z-sidemount system. . .]
Day 8 out of 10 today on the open water wrecks, 45 to 60 meters deep, of the IJN Fourth Fleet Anchorage in Chuuk Lagoon, and my Razor 2 buddy has switched back to conventional backmount for six of those days (he's very impressed with the sturdier wing & harness attachment design of the Z-system). . .
The 9kg/20lbs wing is just enough lift for two 11L (AL80's) tanks, and two 4.2L (AL30's) deco bottles in tropical 29deg C waters, on the surface pre-dive before descent, with a skin suit and no added lead weights.